1. Field of the Invention
The present invention relates to a method for manufacturing lithium-manganese (Lixe2x80x94Mn) oxides which are used for a positive electrode material of Li secondary battery, and more particularly, to a method for manufacturing Lixe2x80x94Mn oxide powders, in which a battery having a high productivity, a large capacity, and a long life cycle is obtained, since powders having an excellent crystallization can be produced by heat-treating for a considerably shorter time than those of conventional methods.
2. Description of the Related Art
In general, there are two technical matters to be solved in relation to a capacity and a lifetime of a Li secondary battery.
Firstly, a state transition phenomenon occurring in the process of charging and discharging a battery may reduce a capacity and lifetime of the Li secondary battery. In the case that a Li secondary battery is charged, lithium (Li) existing in LixMn2O4 (x=1) powders being a positive electrode material is extracted and dissolved in an electrolyte, and the dissolved lithium ions are moved to carbon or graphite being a negative electrode. Meanwhile, in the case that the Li secondary battery is discharged, lithium is separated from carbon and inserted into a crystalline lattice of the LiMn2O4 powders again. Here, in the case that x greater than 0.5 in LixMn2O4, lithium exists as a single phase in which a lithium content is successively varied, so that a crystalline structure is continuously varied at the process of inserting and extracting lithium. However, in the case that x less than 0.5 in LixMn2O4, a lithium content is separated into two different states, that is, xcex-MnO2 (x=0) and Li0.5Mn2O4 (x=0.5). As a result, the lithium insertion and extraction process accompanies a state transition of xcex-MnO2 and Li0.5Mn2O4, to thereby cause the crystalline structure to be severely varied. In this process, since a part of structure in the lattice of the LiMn2O4 powders is destroyed at the time of insertion and extraction of lithium, or Mn+3 ions are dissolved into the electrolyte, a lifetime of the positive electrode is lowered.
It has been found that the above problem cannot be solved by altering conventional processes. That is, both a solid-state reaction method and a sol-gel method reveal the above phenomenon. In order to prevent the lifetime of the positive electrode from being shortened, it is the most essential method to substitute an ion (or ions) such as Li+, Co+2, Cu+2 and Ni+2 whose oxidation number is lower than that of manganese into a manganese position. However, in this case, a Mn+3 ion concentration decreases by valence of the substituted ion and thus the number of the ions which can be participated in the Mn+3 ←xe2x86x92Mn+4 transition becomes small. As a result, a theoretical capacity of a battery decreases as many as the number of the state transition ions decreases. Therefore, it has been recognized that a somewhat reduction in capacity is inevitable in order to improve lifetime of a battery.
Secondly, nano-scale impurities of Mn2O3 and Li2MnO2 which are not found by an X-ray diffraction (XRD) method or an electron microscope, remain in a material at the manufacturing process, which impurities cause lowering of the lifetime during use. Finally, it is the key technique in producing powders having no foreign matter and an excellent crystallization.
Conventionally, for this reason, a solid-state reaction method or a sol-gel method has been used in order to produce the large amounts of powders having no foreign matter and an excellent crystallization. In the case of the solid state reaction method, LiCO3 or LiOH.H2O and MnO2 are well mixed and maintained for 150 hours at 400xc2x0 C. or so, and then maintained again for 24 hours or more at 750xc2x0 C., in order to enhance crystallization. This conventional solid-state reaction method gives an excellent crystallization but requires a long thermal treatment time, to accordingly cause the larger particle size. Thus, it is difficult to use it at the high current condition.
A sol-gel method is mostly used in order to obtain powders having uniform fine particles. Among various sol-gel methods, a method proposed in the Korean Patent Application No. 2000-5210 by the same inventor of this invention is most excellent. This method uses citric acid and ethylene glycol in order to make metal ions uniformly distributed. Here, spontaneous combustion occurs in the manufacturing process and thus crystallization occurs to a certain degree. As a result, although the thermal treatment time is shortened in this sol-gel method, the powders of the same property as that of the powders produced by the solid-state reaction method are obtained.
If the above impurities such as Mn2O3 and Li2MnO2 are removed to a further lower level, it is expected that we can obtain powders having a more excellent property. However, for this purpose, the content of ethylene glycol should be increased considerably. In this case, the ethylene glycol, volatilized during vacuum drying, is condensed on the inner wall of a vacuum line to thereby block the hose. Thus, the vacuum line should be cleaned regularly, to accordingly shorten a facility running time and increase a maintenance fee. Furthermore, since lithium ions are volatilized in a fashion combined with the ethylene glycol, the composition of the powders is not maintained uniformly unless a vacuum drying process is carefully controlled, to accordingly cause a non-uniform property.
To solve the above problems of a conventional solid-state reaction method or a combustion synthesis method, it is an object of the present invention to provide a method for manufacturing a Lixe2x80x94Mn oxide in which glycine is used instead of citric acid and ethylene glycol which are used in a combustion synthesis process and an amount of nitric acid is adjusted.
It is another object to provide a method for easily synthesizing fine LiMn2O4 powders having an excellent crystallization, industrially.
To accomplish the above object of the present invention, there is provided a method for manufacturing LiMn2O4 powders for use in a lithium secondary battery positive electrode, characterized in that oxide or carbonate is dissolved in a nitric acid solution, a spontaneous combustion is performed using glycine, and then the resulting matter is put into a reaction furnace in order to calcine that for a short time.